Untangling Geobacter sulfurreducens Nanowires

Y e et al. (1) present a model for how protein nanowires contribute to the current production of Geobacter sulfurreducens bio ﬁ lms. They state that “ In this study, we examined all three nanowires in the anode bio ﬁ lm of G. sulfurreducens with the goal of de ﬁ ning both their structural and conductive contributions. ” (1). However, Ye et al. (1) did not examine any nanowires in anode bio ﬁ lms, and the only imaging they provided suggested that the strain of G. sulfurreducens that they studied did not express two of the three nanowire types under consideration. Furthermore, the authors misrepresent previously published studies of their own, as well as other investigators.

to account for Liu's prior results (7), as well as the results of other investigators, that have reported proper localization of outer surface cytochromes, but diminished current production in G. sulfurreducens strains expressing poorly conductive pili.
Liu et al. (2) previously reported that DPilB strains of G. sulfurreducens and G. metallireducens were defective in current production (Fig. 1F). Yet now, Ye et al. (1) claim that a DPilB strain produces ca. 90% as much current as wild-type. This reversal in the phenotype reported is remarkable because it has a major impact on the nanowire model. If, as the authors claim, deleting the gene for PilB specifically prevents e-pili expression, then the phenotype that their lab originally reported (2) refutes their claim that e-pili had a minor role in electron transfer through biofilms. An explanation for their change in the phenotype reported is required.
Ye et al.
(1) presented a model in which OmcZ nanowires coursing throughout currentproducing biofilms were the primary conduit for long-range electron transport. This conclusion was based on the finding that deleting the gene for OmcZ greatly diminished current production. Ye et al. (1) fail to note that this same phenotype was reported long ago (11) and that, based on those results, the localization of OmcZ was intensively investigated (12). Those studies demonstrated that OmcZ was specifically localized at the biofilm-anode interface and that OmcZ did not appear to be organized in filaments (12). Thus, the model of Ye et al. (1) was unfounded because they did not provide data to refute the earlier studies on OmcS. The gel image with an arrow is from reference (2) with permission. (D) Heme-stained proteins in the biofilm matrix from strain Tyr3, a mutant in which the tyrosines in the pilin monomer were replaced with alanine. The arrow designates the band for OmcZ. The gel image with an arrow is reprinted from reference (8) (publisher permission was not needed). (E) Heme stained SDS-PAGE of outer surface c-type cytochromes from G. sulfurreducens control strain and strain PA, a strain expressing poorly conductive pili. The gel image with band labeling is from reference (7) (publisher permission was not needed). (F) Current production of G. sulfurreducens and G. metallireducens control strains, DPilB strains, and DPilA strains. The data image is from reference (2) with permission. Descriptive labeling on original images was modified for consistency.
Letter to the Editor mBio OmcZ localization, and as noted above, Ye et al. (1) did not provide evidence that their strain of G. sulfurreducens could express the OmcZ nanowires emanating from cells that their model required. Word limit restrictions prevent full discussion of the failure of Ye et al. (1) to properly present all the data, including contradictory data from their prior studies, that refute their model. They misrepresent recent studies (13) that questioned the rationale behind generating DPilB mutants by suggesting that those studies were conducted on a G. sulfurreducens strain not relevant to their strain without acknowledging that the prior studies also analyzed a DPilB mutation made in the type strain background. Ye et al. (1) also fail to properly attribute previous key discoveries which they imply are their own, such as the fact that e-pili can also play a structural role in biofilm formation (14), as well as the major importance of OmcZ and lesser role of OmcS in high-density current production and biofilm growth on anodes (11).